Loudspeaker with feedback signal generator



1, 1967 's. H. DE KONING 3,334,184

LOUDSPEAKER WITH FEEDBACK SIGNAL GENERATOR Filed May 26, 1964 INVENTOR. STEPHANUS H.DE KONING BY m/ AGENT United States Patent 3,334,184 LOUDSPEAKER WITH FEEDBACK SIGNAL GENERATOR Stephanus Hendrik de Koning, Emmasingel, Eindhoven,

Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed May 26, 1964, Ser. No. 370,262 Claims priority, application Netherlands, June 21, 1963, 294,443; Sept. 26, 1963, 298,464 Claims. (Cl. 179-1) The invention relates to a loudspeaker arrangement in which the movable portion of the loudspeaker is connected to a transducer for producing a voltage corresponding to the acceleration of the said movement. It

is a particular object of the invention to provide a loudspeaker arrangement suitable for delivering a voltage which may be fed back degeneratively to an amplifier controlling the loudspeaker.

It has been suggested previously to couple the movable portion of the loudspeaker to an electromechanical transducer producing a voltage depending upon the loudspeaker movement. This arrangement always involved the difficulty that in such negative feedback undesirable oscillations are generated which indicate that undesirable phase shifts are produced between the electric current supplied to the loudspeaker and the voltage taken from the transducer. If the said phase shift should reach the value which is critical for the production of spurious oscillations only at the very high and very low frequencies, this disadvantage might be obviated by means of electric networks. It has been found, however, that phase shifts also occur within the reproducing band of the loudspeaker owing to resonance phenomena of the component parts joined to the loudspeaker. In practice elimination of the said phase shifts by means of electric networks proves to be highly complicated and sensitive to a variety of disturbances.

It is the object of the present invention to provide a loudspeaker in which the said disadvantages are obviated. The invention is characterized in that the transducer is secured to the apex of a conical member the base of which is attached to the device for driving the movable portion of the loudspeaker.

The invention will be described in detail with reference to the drawing which shows an embodiment of an arrangement in accordance with the invention and the associated circuit arrangement.

The loudspeaker in accordance with the invention comprises a movable portion in the form of a cone diaphragm 1 which is driven by a coil former 2 which is attached to the said diaphragm and on which a loudspeaker coil 3 is provided. The coil former 2 together with the coil 3 are disposed within a magnet system shown diagrammatically, the arrangement being such that currents supplied to the loudspeaker coil 3 cause axial forces to be applied to the coil former so that the cone diaphragm is set into motion. The said currents are produced by means of a loudspeaker amplifier 4 to which audio input signals from a source 5 are applied.

Since generally there is no purely linear relationship between the sound pressure produced by the moving cone diaphragm and the signal of the source 5 it is desirable to improve the said relationship by the provision of negative feedback. For this purpose there is attached to the cone diaphragm a piezoelectric transducer 6 upon which an inertia proportional to the acceleration of the loudspeaker cone diaphragm is exerted. The transducer is preferably made of a crystal material and hence is very hard. If the loudspeaker is accommodated in the wall 9 of an otherwise closed cabinet, for oscillations having a wavelength greater than one half of the circumference of the loudspeaker cone diaphragm the transducer pro- Patented Aug. 1, 1967 ice duces a voltage proportional to the square root of the radiated sound energy. At shorter wavelengths the voltage increases in proportion to the frequency in the case of constant sound energy. In order to allow for'the said effects, it is sufiicient to include a simple network 7 in the negative-feedback circuit, which network integrates the oscillations at shorter wavelengths in order that the negative-feedback voltage returned to the amplifier 4 depends substantially linearly on the square'root of the sound energy over the entire audio frequency range.

Hereinbefore it has been assumed that there are no parasitic phenomena which may cause the voltage of the transducer 6 to deviate from the said relationship. Since the transducer 6 must be joined to the diaphragm 1 in some way, the joining means may provide difficulty in this respect. Due to the hardness of the crystal material from which is constructed transducer 6 it may unintentionally absorb mechanical forces and hence produce corresponding electric voltages.

According to the invention the transducer 6 is secured to the apex of an axially rigid conical member 8, the base of which is attached to the loudspeaker diaphragm. The conical member points in the direction in which the sound is radiated and consequently has acoustically satisfactory radiation properties. Preferably it is so flexible radially as to prevent the transmission of radial forces from the conical diaphragm 1 to the solid cone apex to which the transducer 6 is secured. At its lower end the conical member is enclosed by a plate 10 to prevent pressure variations in the surrounding air from affecting the transducer.

The invention is based on the recognition that the axial movement of the cone 1 involves radial reaction forces exerted on the diaphragm 1 by the wall 9. These reaction forces reach the base of the conical member 8, attached to the coil former 3, along its entire circumference. Owing to the flexible conical construction chosen, however, they will be materially attenuated before reaching the apex of the conical member 8 so that no pressure corresponding to the said radial forces reaches the transducer 6 and hence the voltage delivered by the transducer 6 corresponds substantially to the axial accelerations only. Thus an important source of distortion in the production of a negative-feedback voltage is avoided.

In a practical embodiment, the loudspeaker was provided with a conical diaphragm 1 having a diameter of 13 cm. and an apical angle of The diameter of the coil former 3 was 2.6 cm. and its length 2.5 cm. The conical member 8 was made of aluminium and had an apical angle of 40. The diameter of the transducer 6 was 0.6 cm. and its length 1.6 cm. Its mass was of the same order of magnitude as that of the coil 3. By connecting the conical member 8 to earth the induction of ripple voltages in the transducer 6 was also avoided.

What is claimed is:

1. A loudspeaker for translating electrical signals into acoustic waves comprising, a radiating cone having its outer portion attached to a fixed support, first means attached to the inner portion of said cone for causing said cone to oscillate in response to electric signals when supplied to said means, a conical support means having its base attached to the said first means for movement thereby and its apex free standing, and a transducer mounted within the conical support means and attached to the support means at the apex for providing a voltage corresponding to the cone acceleration.

2. A loudspeaker as set forth in claim 1 in which the apex of the conical support means points substantially in the direction of propagation of the acoustic waves.

3. A loudspeaker as set forth in claim 1 in which the conical support means is constructed of metal which is at the base flexible in the radial direction and is provided with an electrically conductive path to ground.

4. A loudspeaker for translating electrical signals into acoustic Waves comprising, a radiating cone having its outer portion attached to a fixed support, first means for establishing a positionally fixed magnetic field, second means located within the magnetic field and including a voice coil attached to the inner portion of said cone for causing said cone to oscillate in response to electric signals When applied to said voice coil, a conical support 10 means having its base attached to said second means for movement thereby and its apex free standing, and a transducer mounted within the conical support means and References Cited UNITED STATES PATENTS 8/1960 Clements 179-1 11/1961 Bekey 179l KATHLEEN H. CLAFFY, Primary Examiner.

R. P. TAYLOR, Assistant Examiner. 

1. A LOUDSPEAKER FOR TRANSLATING ELECTRICAL SIGNALS INTO ACOUSTIC WAVES COMPRISING, A RADIATING CONE HAVING ITS OUTER PORTION ATTACHED TO A FIXED SUPPORT, FIRST MEANS ATTACHED TO THE INNER PORTION OF SAID CONE FOR CAUSING SAID CONE TO OSCILLATE IN RESPONSE TO ELECTRIC SIGNALS WHEN SUPPLIED TO SAID MEANS, A CONICAL SUPPORT MEANS HAVING ITS BASE ATTACHED TO THE SAID FIRST MEANS FOR MOVEMENT THEREBY AND ITS APEX FREE STANDING, AND A TRANSDUCER MOUNTED WITHIN THE CONICAL SUPPORT MEANS AND ATTACHED TO THE SUPPORT MEANS AT THE APEX FOR PROVIDING A VOLTAGE CORRESPONDING TO THE CONE ACCELERATION. 